In everyday language: “Accelerate the rhythm” has the value of “do we increase the tempo?” Or even “We will have to improve the pace!” But in music, not everything is to be put in the same basket. Rhythm and tempo are not the same thing.
Tempo is an Italian word, and it means “time”! The tempo defines the speed at which we will play music. In other words, it’s a pulsation scale: It’s regular, homogeneous, it can go more or less quickly, move elastically, but in any case, it structures the speed of execution of the piece of music.
Today, thanks to the metronome, we can quantify this speed, with an indication on the score, and this figure, the unit used therefore, is the beat per minute! To give you an idea, a movement at a fast tempo will be around 140 bpm, while a slow tempo will be indicated as 60 bpm. To indicate the tempo, composers do not only use numbers: Allegro, Andante, Largo, Vif, Presto, Moderato, the tempo indications are numerous and varied. These indications give a general indication of the speed to choose to play the piece. It is then up to the interpreter to give it a particular meaning.
From Beethoven’s compositions, we find numerical indications, the metronome having been invented at the time, but Beethoven himself often went back on his numbers: the first metronomes were perhaps not the most reliable…
The numerical indications of tempo are in any case to be taken with precaution, Brahms himself evoked the fact that the number was valid for the beginning of the piece, to have an overall speed, and that then the interpreter could lend himself to freedoms .
And what about the rhythm so ? That’s another matter! The tempo defines an execution speed. As for the rhythm, it is rather the indication of the composer which allows the musician to know how to chant the written text. It’s a bit as if the poet wrote for us above the text the places where the words, or the syllables, had to be gathered or separated, said to be grouped together quickly, or with space. In short, which word is long, which word is short, a bit like Morse code, or, precisely, the scansion of the text. Rhythm is how the melody is shaped, which note is long, and which is played in a very short time, often with other short notes following.
Yet another metaphor for you to understand, the rhythm is a bit like the relief of the score.
In summary, we can say that the tempo is the overall speed of the piece while the rhythm is the indication of the duration of a note played at a given moment. Consequently, a “fast” rhythm, that is to say many notes in a short time, played in a fast tempo, is more difficult to execute than this same rhythm in a slow tempo. There are, in fact, two notions of time management in music.
It is essential to understand this because time management, in classical music, is precisely the heart of the matter. A melody can be sublimated if its rhythm is executed with good management of time, that is to say of the tempo. I would add that the rhythm is determined, fixed by the composer on the score, and therefore immutable. You don’t change a written rhythm. The tempo, on the other hand, is malleable, and is the source of the possibility of an interpretation! Not the only one, of course, but one of the main ones! This is how you can play “rubato”, press, or on the contrary apply the brakes, thus making the musical text speak.
In summary, we play the written rhythm, and we rely on the tempo to give expression and interpret the music! Tempo and rhythm, two very different things, but very close too.